1. Field of the Invention
The present invention relates to a precision lathe for carrying out machining of grooves on a roll, and more particularly to a precision roll turning lathe which can not only machine circumferential grooves in the peripheral surface of a roll but can also machine axial grooves in the roll surface with high precision.
2. Background Art
Roll grinders and roll turning lathes are generally used as machine tools for machining rolls. A roll grinder includes a headstock, a tail stock, and a carriage which is provided with a grinding wheel.
Such a roll grinder can carry out grinding with a grinding wheel of the peripheral surface of a roll and, in addition, machining of grooves in the peripheral surface. Japanese Patent Laid-Open Publication No. 2003-94239 describes a roll grinder which is provided, in its carriage, with a groove-machining device having a cutting saw blade for groove cutting.
A roll turning lathe is a lathe in which a tool post, having e.g. diamond tools attached thereto, is mounted in a carriage, and is basically used for machining circumferential grooves in a roll by rotating the roll with a headstock and feeding the carriage in the lateral direction (X-axis direction) of the roll. When machining axial grooves in a roll, the carriage is moved at a high speed in the longitudinal direction (Z-axis direction) of the roll while indexing the roll with the headstock (C axis), thereby producing the axial grooves.
The recent progress of control technology has realized ultraprecision machining with a lathe, and it has become possible to machine with a lather a mold for molding an optical lens. For example, the applicant has proposed a vertical lathe capable of machining a mold for molding of a Fresnel lens (Japanese Patent Laid-Open Publication No. 2004-358624). The vertical lathe can machine with high precision V-shaped lens grooves of a mold for molding of a Fresnel lens.
Owing to the widespread use of liquid crystal displays, there is an increasing demand for lens sheets for use in a backlight of a liquid crystal panel. Such lens sheets include, besides the above-mentioned Fresnel lens, a lenticular lens sheet, a cross lenticular lens sheet, a prism sheet, etc.
Molding a lenticular lens sheet, a cross lenticular lens sheet or a prism sheet by means of an extruder using roll-shaped mold has recently been studied.
When such a roll-shaped mold for molding of a lens sheet is machined by using a roll turning lathe, it is necessary to machine with precision circumferential grooves (transverse grooves) and longitudinal grooves in the peripheral surface of a roll.
Machining of the transverse grooves is carried out by feeding a diamond tool in the radial direction of the roll while rotating the roll at a high speed by means of the headstock of the lathe, as described above. Owing to the high-speed rotation of the roll, the vertical grooves can be machined at sufficiently high cutting speed, providing a good machined surface.
On the other hand, machining of the longitudinal grooves is carried out by feeding the carriage of the lathe in the longitudinal direction of the roll after indexing the roll with the headstock. Such machining involves the problem of insufficient cutting speed. It is to be noted in this regard that while an ideal cutting speed is about 300 m/min in the case of a roll plated with copper or nickel, the moving speed of a high-speed carriage would be about 10 m/min at most. Thus, a high-speed carriage, even when moved at the highest speed, will fail to afford cutting speed enough to provide a high-precision machined surface, in particular such an ultraprecision machined surface as required for a mold for molding a lens sheet.